The invention relates to an automatic speed ratio control system which controls the speed of an stepless transmission of an automotive vehicle so that a given engine condition is achieved.
The output torque T.sub.E of an engine is related to the torque which is dissipated by the running of the vehicle as the following equation, assuming no torque loss: EQU T.sub.E =(J.multidot.n.sub.2 +A.multidot.n.sub.2.sup.2 +B)x e (1)
where "e" represents a speed ratio, "n.sub.2 " represents the rotational speed of an output shaft, n.sub.2 represents dn.sub.2 /dt, "J" represents the output inertia, "A" represents windage loss and "B" represents grade resistance. Since the speed ratio "e" presents the ratio of the rotational speed "n.sub.2 " of the output shaft to that "n.sub.1 " of the input shaft, we have EQU n.sub.2 =e.multidot.n.sub.1 ( 2)
On the other side, the output torque T.sub.E of an internal combustion vehicle engine especially a gasoline engine has a characteristic as shown in FIG. 10 and is represented by the following equation (3): EQU T.sub.E =-K(.theta.).multidot.n.sub.1 +.alpha.(.theta.) (3)
Accordingly, we have the following equation (3)": ##EQU1##
Prior to the invention, the speed ratio e was controlled on the basis of the following equation (4): EQU e=K.sub.1 .multidot.(n.sub.1 -n.sub.o) (4)
wherein "n.sub.o " represents a target speed of the engine and "K.sub.1 " represents a gain constant.
Comparing the equations (4) and (3)", we can understand that ##EQU2## of the equation (3)" is represented by "n.sub.o " of the equation (4), and ##EQU3## of the equation (3)" is represented by the gain constant "K.sub.1 " of the equation (4). The gain constant "K.sub.1 " is so determined as to fit with the normal running condition of the automotive vehicle. A prior system for controlling the speed ratio e on the basis of the equation (4) was disclosed in U.S. Pat. No. 3,803,843,. However, the system may cause an abrupt acceleration of the engine beyond the target speed at the starting (n.sub.2 .apprxeq.0) of the automotive vehicle when the vehicle has a wide range of "n.sub.2 " and thus the variation range of the speed ratio "e" is wide. Because the alteration rate "e" is not enough to increase the speed ratio "e" to transmit the output torque T.sub.E to the output shaft of the stepless transmission sufficiently. The insufficient speed ratio "e" at the starting may provide a driver with an unpleasant feeling and cause a fall of an acceleration ability of the automotive vehicle.
In another systems such as disclosed in U.S. Pat. No. 3,237,398, the alteration rate "e" is controlled in response to the throttle opening ".theta." on the basis of the following equation (5); EQU e=.alpha..multidot..theta. (5)
However, the increase of the output torque T.sub.E of the engine may delay from the alteration of the throttle opening ".theta.". Thus the alteration rate "e" may lead the increase of the output torque T.sub.E. U.S. Pat. No. 3,283,606 discloses a speed ratio control system having a slide valve member 34 and a fluid-flow retarding device 39 for controlling an actuator. The resistance of the fluid-flow retarding device 39 is adjusted as a function of a pump disc inclination which is controlled by the actuator. Thus the alteration rate of the speed ratio is controlled to increase and decrease when the speed ratio is low and high respectively.